1. Field of the Invention
The present invention relates to electroanalytic transducers used for quantitative electrochemical analysis of chemical substances and more particularly to apparatus and a method for insulating preselected portions of transducer electrodes from electrolyte.
2. Description of the Prior Art
Transducers of the type used for quantitative electrochemical analysis of chemical substances are well known in the art and generally include a working or sensing electrode having a defined, or electroanalytically effective, surface portion for contact with an electrolyte and an insulator that limits the exposed surface of the working electrode. For amperometric analytical operation, the working electrode in a transducer-type cell arrangement is polarized by a constant DC voltage to furnish a current whose steady state magnitude is proportional to the activity of the chemical substance of interest. Transducers of this type and their operation and uses are discussed in detail in the following illustrative U.S. Pat. Nos. 2,913,386, 3,071,530, 3,223,608, 3,227,643, 3,372,103, 3,406,109, 3,429,796, 3,514,658 and 3,622,488. Generally, these prior art transducers are electrochemical cells in which a suitable electrolyte contacts the working electrode, the counter electrode, and the insulator that separates the electrodes, and prevents direct electric currents from flowing between the electrodes so that any current which is permitted to pass is ionic current in the electrolyte arising from electrochemical phenomena at the working electrode and the counter electrode. Important examples of this type of electrochemical transducer are the membrane enclosed polarographic devices which are widely used for the measurement of the concentrations of gases such as oxygen, chlorine and sulfur dioxide in a fluid, such as water, a gas mixture and the like. A recognized fault or deflect common to all conventional electrochemical transducers is the presence of an unwanted contribution to the total current that is unrelated to the activity of the substance to be analyzed. This unwanted contribution is manifested in two ways during operation: as an excess exponentially decaying transient contribution initiated by switching the current on; and as a residual contribution which remains after the chemical substance of major interest is excluded from the system. These defects restrict the application of the transducer to a range of activities of the chemical substance to be analyzed which are greater than a minimum detectable limit, and introduce a waiting period for the signal to stabilize before measurements may be begun. Furthermore, it is commonly observed that the magnitude and period of stabilization of the transient signal and the size of the residual current increase with the age of the transducer.
In accordance with the present invention, it has been found that an essential and common cause of the aforementioned defects of prior art transducers is a penetration phenomenon characterized by unintended contact of portions of the working electrode with the electrolyte. Thus, those portions not intended for contact with the electrolyte become exposed to interaction with the electrolyte or constituents thereof, thus changing the electroanalytically effective surface portion of the working electrode and forming an uncontrolled "gap" or access channel. While not wishing to be bound by any theory, this phenomenon can be explained by an unexpectedly strong tendency of a liquid electrolyte or electrolyte portion including the ions present or formed during operation, for example, water with or without the electrolytic solute in the case of aqueous electrolytes, to creep into the junction or interface between metal and common insulating materials. The charged state of the electrode during operation of the transducer does increase the creep tendency of electrolyte penetration into the metal/insulator interface. As a consequence of this penetration, the available area of the electrode which can interact with the electrolyte and at which electrode reactions can take place may be greatly extended, even though the additional area may be accessible only through, a highly resistive electrolytic conductor. Double layer capacity charging of the extended electrode surface will occur, and the charging current flows through the electrolytic resistance of the uncontrolled gap or access channel, thus increasing the magnitude of the unwanted current contribution in prior art transducers. It is important to note that the aforementioned prior art defects are present even in transducers in which the electrode/insulator junction commonly is assumed to be impermeable to liquids, for example, a strong adhesive bond obtained by cementing or curing a thermosetting polymer in contact with the metal electrode. Attempts to improve the adhesive bond, for example, by using polymers with polar groups and other methods for improving adhesive bonding, fail to show a significant improvement.
In accordance with the present invention substantial improvements in the performance of electroanalytical transducers are obtained over the prior art.